1. Field of the Invention
The present invention relates to a power transmission apparatus for helicopter and more particularly to a power transmission apparatus for transmitting torque and rotational movement generated in an engine to a main rotor of a helicopter.
2. Description of the Prior Art
In the helicopter, since a Mach number of the rotary wing can not be raised so high, a power transmission apparatus or simply referred to as so-called "transmission" needs a large reduction gear ratio, especially in case where a gas turbine engine is used. FIG. 6 is a schematic view showing a typical arrangement of the power train system in the helicopter, in which an output shaft 2 of an engine 1 is connected to a power transmission apparatus 3. A drive shaft 4 for driving a main rotor 5 and a tail rotor drive shaft 6 for driving a tail rotor 7 are extended from the power transmission apparatus 3. Further, power is transmitted from the power transmission apparatus 3 to auxiliary devices too such as generators, hydraulic pumps and the like.
Because of the nature of components for aircraft, compactness and light weight are essential factors in designing the power transmission apparatus of the helicopter. To attain this requirement, for example a literature (A 2400 Kw Light Weight Helicopter Transmission With Split-Torque Gear Trains: ASME 84-DET-91) discloses a construction using a pair of gear trains. Namely, as shown in FIG. 4, a gear wheel 9 is fixedly secured to a drive shaft 4 of main rotor and a pair of pinions 10 meshes with the gear wheel 9. Further, a pair of medium gears 11 is coaxially mounted on each of pinions 10 and meshes with a pinion 12. In a gear train thus constituted, an engine torque is transmitted to a shaft 13 of the pinion 12 through a helical bevel gear 14 and further it is transmitted to the gear 9 through the pair of medium gears 11 and the pair of pinions 10. Thus, since a torque is splited to each of pinions 10, the power transmission apparatus according to the prior art can attain compactness and light weight. In this example of prior art as illustrated in FIG. 4, another pair of pinions meshes with the gear 9 oppositely thereto. This pair of pinions is for another power source and has no relation to the description of the present invention.
However, in this prior art there is a disadvantage that an equal torque splitting is realized only when all of gears constituting the gear train system meet individual specifications accurately. In an actual world, because of manufacturing tolerances, the equal torque splitting is very difficult to be realized.
In order to solve this problem, mechanisms for equalizing input torque have ever been proposed. For example, a literature (Split Torque Transmission Load Shearing: NASA TM-105884) discloses a torque split apparatus in which, as shown in FIG. 5, a helical pinion 16 is provided on an input shaft 15 and a pair of medium helical gears 17 meshes with the helical pinion 16. Shafts 18 of the medium helical gears 17 are sheathed with a pinion 19 meshed with a gear 9 respectively. These shafts 18 are constituted so as to be axially slidable and upper ends thereof are connected with a balance beam 20 respectively. The balance beam 20 acts as balancing a thrust force produced in one shaft 18 with a thrust force produced in another shaft 18, whereby a torque is equally distributed.
However, in this torque split apparatus since the shafts 18 are constituted to be axially slidable when an unbalance occurs in making torque distribution, other types of gears than spur gear can not be used for the pinion 19. Therefore, in this prior it is difficult to achieve a weight reduction by making use of a more compact gear such as helical gear or double-helical gear.
Further, there is another problem in this torque split apparatus such as needing a complicated mechanism like a balance beam 20 including two movable elements therein, as illustrated in FIG. 5.